Description

The main figure shows eight records of local temperature variability on multi-centennial scales throughout the course of the Holocene, and an average of these (thick dark line). The records are plotted with respect to the mid 20th century average temperatures, and the global average temperature in 2004 is indicated. The inset plot compares the most recent two millennium of the average to other high resolution reconstructions of this period.

At the far left of the main plot climate emerges from the last glacial period of the current ice age into the relative stability of the current interglacial. There is general scientific agreement that during the Holocene itself temperatures have been quite stable compared to the fluctuations during the preceding glacial period. The average curve above supports this belief. However, there is a slightly warmer period in the middle which might be identified with the proposed Holocene climatic optimum. The magnitude and nature of this warm event is disputed, and it may have been largely limited to summer months and/or high northern latitudes. [1]

Because of the limitations of data sampling, each curve in the main plot was smoothed (see methods below) and consequently, this figure can not resolve temperature fluctuations faster than approximately 300 years. Further, while 2004 appears warmer than any other time in the long-term average, an observation that might be a sign of global warming, it should also be noted that the 2004 measurement is from a single year (see Image:Short Instrumental Temperature Record.png for comparison to other years). It is impossible to know whether similarly large short-term temperature fluctuations may have occurred at other times but are unresolved by the resolution available in this figure. The next 150 years will determine whether the long-term average centered on the present appears anomalous with respect to this plot.

Since there is no scientific consensus on how to reconstruct global temperature variations during the Holocene, the average shown here should be understood as only a rough, quasi-global approximation to the temperature history of the Holocene. In particular, higher resolution data and better spatial coverage could significantly alter the apparent long-term behavior (see below for further caveats). For another estimate of Holocene temperature fluctuations, see: [2]

While any conclusions to be drawn from the long-term average must be considered crude and potentially controversial, one can comment on a number of well established inferences from the individual curves contributing to the average. First, at many locations, there exist large temperature fluctuations on multi-centennial scales. Hence, climate change lasting for centuries appears to be a common feature of many regions. Assuming the timing information from these records is reasonably accurate, it appears that in many cases large changes at any particular site may occur without correlating to similarly large changes at other sites. Secondly, it is also notable that different locations appear to take different amounts of time to reach typical Holocene conditions following the last glacial termination. Scientists generally agree that warming concluded in the far Southern Hemisphere earlier than in most other regions. In part, the prolonged climate change may be related to prolonged changes in sea level, which took till roughly 7000 years ago to reach near modern levels. Some of the differences may also reflect timescale uncertainties.

Temperature Record Series

This figure is part of series of plots showing changes in Earth's temperature over time.

Methods

To construct this plot, eight data sources (listed below) were selected on the basis of good temporal resolution (preferably ~100 years or less per data point) and coverage of the last 12 kyr. Seven of the eight specifically reported temperature and were used as is. The other one reported an unscaled temperature proxy and was scaled as described in Notes below. Each curve was smoothed by a Gaussian weighted filter to produce a history of the Holocene temperature variations at that site with approximately 300 year resolution (one exception, see notes). These smooth curves were adjusted to have the same mean over the interval 100-6000 years BP. The average of these curves was then constructed, and the alignment relative to modern day determined by comparing the average over the interval 250-1900 AD relative to the three short-term proxies shown in inset (for details on those plots, see: Image:2000 Year Temperature Comparison.png). Note that the short-term proxies are not at all used in constructing the average itself.

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Caveats

A typical modern temperature construction involves 10s or 100s of records to ensure reasonable global coverage. Because the average shown here involves only 8 records, it is entirely plausible that significant contributions to temperature variation are being overlooked because certain large regions (e.g. Asia) are not being sampled.

Different regions have different sensitivity to global temperature variations, so one can reasonably argue that a true global average reconstruction requires scaling the different records to match local sensitivity. Since no scientific consensus exists for how to do this over this time scale, no attempt was made to do this.

Given the limited spatial sampling, it is unclear whether the slightly warmer period during the Holocene climatic optimum corresponds to a statistically significant difference.

Because the Davis et al. pollen reconstruction is based on measurements across many sites in Europe, it is more reasonably described as a regional rather than a local temperature measurement. Similarly, it shows considerably less short-term variability than most other sites.

A small number of records, not used in this study, have been interpreted as indicating much larger temperature variation during the Holocene (5+ °C) than the records represented here. In many cases, critics have interpreted these changes to reflect seasonal, rather than annual variations in temperature, or very unusual local changes. However, the possibility exists that the current reconstruction underestimates long-term variability.

During the earliest parts of this record, the timing uncertainty on some records may become substantial, potentially exceeding a couple hundred years. This could have the result of causing correlated features to fall out of alignment.

Notes

The time scale for sediment core ODP 658 was converted from the listed Carbon-14 "equivalent" time scale to calendar years by using the INTCAL04 calibration curve (Reimer et al. 2004).

Because of poorer data resolution, the sediment core PL07-39PC was smoothed to an approximately 500 year average.

δ18O temperature proxy from the Kilimanjaro ice core was converted to °C by scaling its variance from the interval 100-6000 years BP to match the average variance reported in the other local records during the same period.